An important proportion of iron oxides for ironmaking are provided in a pellet shape. To manufacture the pellets, an iron ore concentrate is agglomerated on one or several balling devices and the agglomerated balls are fired in an induration furnace, such as a moving grate furnace or a grate kiln, to induce diffusion bonding, thereby increasing their mechanical properties for their handling and transportation to a reduction site.
In the induration furnace, the agglomerated balls are first dried in a drying zone to remove their water content. They can then be pre-heated in a pre-heating zone in order to gradually increase their temperature to avoid thermal shock. The agglomerated balls are then indured in a high temperature induration zone to create physical links between the particles and, consequently, increase their mechanical properties. Finally, the pellets are cooled in a cooling zone to obtain pellets at a temperature suitable for subsequent handling.
The drying and diffusion bonding processes occur mostly by heat transfer through forced convection, i.e. the air circulating in the drying and induration zones is heated and heat is transfer to the pellets. The induration of the agglomerated balls involves high energy consumption in the drying and induration zones. For instance, in the induration zone, the air circulating around the agglomerated balls can reach temperatures up to 1350° C.
Air heating is typically accomplished using heavy oil or natural gas burners.
Heavy oil burners used for this purpose generally use pressurized air (i.e. air with a pressure greater than atmospheric pressure) to force fuel through a nozzle and atomize it into a fine spray. The burner is inserted into the induration furnace where the spray is ignited into a flame which heats the air directly in the induration furnace. As can be appreciated, the burner is exposed to very high temperatures. Existing heavy oil burners are therefore provided with a cooling mechanism in order to prevent damage during operation and to control the flame. This cooling mechanism involves using low pressure air (i.e. air with pressure greater than atmospheric pressure, but less than that of the pressurized air used for atomization) to regulate the temperature of the burner and control the flame.
With reference to FIGS. 1A to 1C, an oil burner 1 of the prior art is shown. The burner 1 has a metal-based body 3 shaped as a “spear” which comprises an elongated portion 5 and a nozzle 7. The elongated portion 5 houses three concentric flow lines 9, 11, 13 for transmitting fluid to the nozzle 7. In the center is a fuel line 9 for carrying heavy oil fuel 9a in liquid form to the nozzle 7. Around the fuel line 9 is an atomization gas line 11 which carries air at high pressure 11a to the nozzle 7. Finally, around the atomization gas line 11 is a cooling air line 13 which carries air at low pressure 13a to help cool the spear and control the flame at a nozzle outlet 15.
As can be appreciated, the fuel 9a and atomization air 11a are mixed inside the nozzle 7 to form an atomized fuel mixture 15a at the outlet 15 of the nozzle 7. Meanwhile, the cooling air line 13 cools the spear by circulating air 13a along outer sidewalls 17 of the spear body 3. The air is blown at low pressure along the outer sidewalls 17 before eventually exiting around the nozzle 7. Blowing the air 13a in this fashion also allows controlling the flame at the outlet 15.
Although the cooling air 13a helps regulate the temperature of the burner 1 and keep it at a nominal temperature during continuous operation, the air 13a exiting the nozzle 7 causes additional cold air to enter into the induration air. This additional air must also be heated, and heating this additional air requires additional oil consumption which can be considered as being a loss of energy.